Most electronic products (including a television set, a computer monitor, a VCR, etc.) operate in two modes: a normal operation mode where a large amount of power is consumed; and a standby mode where a small quantity of power is consumed while awaiting a normal operation signal. Though electronic products consume a smaller amount of power in the standby mode than in the normal operation mode, they are generally in the standby mode for a longer period of time. Consequently, efforts have recently focused on decreasing the amount of power lost while in the standby mode.
To that end, electronic products employ an auxiliary power supply or reduced output voltage in order to decrease the input power of the standby mode. A reduction in the output voltage, however, necessitates an increase in the number of components. There is a also a limit in the ability to decrease power consumption of the standby mode. Furthermore, an auxiliary power supply raises component costs.
A general SMPS consumes most of its input power when switching a control integrated circuit (IC) and a main switch of its primary side in the standby mode so that the power transferred to the secondary side is reduced, thereby deteriorating efficiency. To solve this problem, a burst mode SMPS operating in a burst mode in which the SMPS performs a switching operation in the standby mode for a predetermined period of time, and then stops the switching operation for a specific period of time, has been proposed. This reduces the number of switching times to increase efficiency.
FIG. 1 shows a conventional burst mode SMPS circuit. As illustrated in FIG. 1, the burst mode SMPS controls output voltages Vo1 and Vo2 according to a feedback circuit 10 that observes the output voltage of the secondary side of the circuit in the normal operation mode. The circuit also contains a switching controller 20 for controlling switching time according to the feedback information obtained by the feedback circuit 10. In the standby mode, the burst mode SMPS circuit operates in a burst mode in which the circuit observes and controls the output voltages according to the turn ratio of a transformer Tx that outputs current to Vcc.
In this conventional burst mode SMPS, however, current flows through a circuit that does not operate in the standby mode, thereby causing power to be unnecessarily consumed. To prevent this, the output voltage of the secondary side can be maintained at the minimum voltage for driving a microcomputer and reduce the output voltage to lower than the output voltage in the normal operation mode. In this case, however, the voltage applied to the switching controller 20 of the primary side cannot satisfy the minimum voltage for operating the switching controller 20.